"It must be deleted," Eugene Kaspersky says he researchers with NSA files.

Sometime in 2014, a group of analysts walked into the office of Eugene Kaspersky, the ebullient founder of Russian cybersecurity firm Kaspersky Lab, to deliver some sobering news.

The analysts were in possession of a cache of files belonging to the Equation Group, an extraordinarily powerful band of hackers that would later be exposed as an arm of the US National Security Agency. But the analysts were worried; the files were classified.

"They immediately came to my office," Kaspersky recalled, "and they told me that they have a problem."

According to him, there was no hesitation about what to do with the cache. "It must be deleted," Kaspersky says he told them.

The incident, recounted by Kaspersky during a brief telephone interview on Monday and supplemented by a preliminary timeline provided by company officials, could not be immediately corroborated.

But it's the first public acknowledgement of a story that has been building for the past three weeks — that Kaspersky's popular anti-virus programme uploaded powerful digital espionage tools belonging to the NSA and sent them to servers in Moscow.

The account provides new perspective on the US government's recent move to blacklist Kaspersky from federal computer networks, even if it still leaves important questions unanswered.

To hear Kaspersky tell it, the incident was an accident borne of carelessness.

Kaspersky was already on the trail of the Equation Group when one of its customers in the United States — Kaspersky referred to them as a "malware developer" — ran at least two anti-virus scans on their home computer after it was infected by a pirated copy of Microsoft Office 2013, according to Kaspersky's timeline. That triggered an alert for Equation Group files hidden in a compressed archive which was spirited to Moscow for analysis.

Kaspersky's story at least partially matches accounts published in The New York Times, The Washington Post and The Wall Street Journal.

But information security experts reading the bits of information dropped by anonymous government officials are still puzzling at whether Kaspersky is suspected of deliberately hunting for confidential data or was merely doing its job by sniffing out suspicious files.

Much of the ambiguity is down to the nature of modern anti-virus software, which routinely submits rogue files back to company servers for analysis.

The software can easily be quietly tweaked to scoop up other files too: perhaps classified documents belonging to a foreign rival's government, for example.

Eugene Kaspersky, CEO, previously said he would give the US software code to soothe spying fearsAlexey SAZONOV/AFP/Getty Images

Concerns have been fanned by increasingly explicit warnings from US government officials after tensions with Russia escalated in the wake of the 2016 presidential election.

Kaspersky denied any inappropriate link to the Russian government, and said in his interview that any classified documents inadvertently swept up by his software would be destroyed on discovery.

"If we see confidential or classified information, it will be immediately deleted and that was exactly [what happened in] this case," he said, adding that the order had since been written into company policy.

An AP request for a copy of that policy wasn't immediately granted.

Kaspersky's account still has some gaps. How did the analysts know, for example, that the data was classified? And why not alert American authorities to what happened?

Coding symbols at the headquarters of Kaspersky in Moscow, 2016KIRILL KUDRYAVTSEV/AFP/Getty Images

Several reports alleged that the US learned that Kaspersky had acquired the NSA's tools via an Israeli spying operation.

Kaspersky declined to say whether he had ever alerted US authorities to the incident.

"Do you really think that I want to see in the news that I tried to contact the NSA to report this case?" he said at one point. "Definitely I don't want to see that in the news."

So did he alert the NSA to the incident or not? "I'm afraid I can't answer the question," he said.

Even if some questions linger, Kaspersky's explanation sounds plausible, said Jake Williams, a former NSA analyst and the founder of Augusta, Georgia-based Rendition InfoSec.

He noted that Kaspersky was pitching itself at the time to government clients in the United States and may not have wanted the risk of having classified documents on its network.

"It makes sense that they pulled those up and looked at the classification marking and then deleted them," said Williams. "I can see where it's so toxic you may not want it on your systems."

As for the insinuation that someone at the NSA not only walked highly classified software out of the building but put it on a computer running a bootleg version of Office, Williams called it "absolutely wild."